The year 1905 is often called Einstein’s annus mirabilis (“miracle year”), since during that year he published his trailblazing papers on how light knocks electrons off metals (the “photoelectric effect,” which spawned quantum mechanics and won him the Nobel Prize), on random drifting of particles suspended in a fluid (“Brownian motion”), and on the theory of “special relativity.” While 1905 was indeed a year of wonders for Einstein, he actually had a second annus mirabilis (fifteen months, to be exact) from November 1915 to February 1917. During this period, he published no fewer than fifteen treatises, including the brilliant pinnacle of his work—general relativity—and two significant contributions to quantum mechanics. Modern cosmology, and with it the cosmological constant, were born.

I hope that the evidence presented in chapter 10 has convinced the reader that most probably Einstein never used the expression “biggest blunder.” Moreover, the introduction of the cosmological constant was not a blunder at all, since the principles of general relativity gave the green light for such a term. Thinking that the constant would ensure a static universe was definitely a regrettable mistake but not one that would qualify as a “blunder” of the magnitude considered in this book. Einstein’s true blunder was to remove the cosmological constant! Mind again that removing the term from the equations is tantamount to arbitrarily assigning the value zero to lambda. In doing so, Einstein restricted the generality of his theory—a high price to pay for the conciseness of the equations, even before the recent discovery of the cosmic acceleration.

Simplicity is a virtue when it applies to the fundamental principles, not to the form of the equations. In the case of the cosmological constant, Einstein mistakenly sacrificed generality on the altar of superficial elegance. A simple analogy can help to clarify this concept. When Kepler discovered that the planetary orbits were elliptical rather than circular, the great Galileo Galilei refused to believe it. Galileo was still prisoner to the aesthetic ideals of antiquity, which assumed that the orbits had to be perfectly symmetrical. Physics has shown however that this was an unjustified prejudice. The symmetry involved actually runs much deeper than the mere symmetry of shapes. Newton’s law of universal gravitation states that the elliptical orbits (which are a natural consequence of this law) can have any orientation in space. In other words, the law doesn’t change whether we measure directions with respect to the north, south, or the nearest star—it is symmetric under rotations. When Einstein dubbed the cosmological constant “ugly,” he was equally biased and shortsighted. He should have instead stuck with his initial instinct that a day would come “for us to be able to decide empirically the question of whether or not Λ vanishes,” as he wrote to de Sitter. That day came in 1998.

More than 20 percent of Einstein’s original papers contain mistakes of some sort. In several cases, even though he made mistakes along the way, the final result is still correct. This is often the hallmark of truly great theorists: They are guided by intuition more than by formalism. In a letter he wrote on February 3, 1915, to the Dutch physicist Hendrik Lorentz, Einstein provided his own perspective on mistakes in scientific theories:

A theorist goes astray in two ways: